*4.6. Transparency and Color Parameters*

The transparency of the SA–AG hydrogel film samples was measured at a wavelength of 550 nm by using a spectrophotometer (ONDA-Vis spectrophotometer, V-10 Plus, ONDA, Padova, Italy) according to the method described by Zhao et al. [48].

The surface color analysis of the SA–AG-based films was carried out by using a colorimeter (Konica Minolta, Tokyo, Japan), and represented as L\* (lightness), a\* (red/green), and b\* (yellow/blue). The film samples were placed on the surface of a standard plate (L\* = 100) and the color parameters L\*, a\*, and b\* were calculated. Equation (5) was used to calculate the ∆E (the overall color difference).

$$
\Delta \mathbf{E}^\* = \left[ \left( \Delta \mathbf{L}^\* \right)^2 + \left( \Delta \mathbf{a}^\* \right)^2 + \left( \Delta \mathbf{b}^\* \right)^2 \right]^{1/2} \tag{5}
$$
